it. The focus on CO 2 , here and in the worldwide discussion, is because it has one of
the longer atmospheric residence times among greenhouse gases and has a large
anthropogenic production. Keep in mind, however, that other gases and a complex
skein of land and sea processes are involved in climate control. There are abundant
details in IPCC (2007, Vol. 1).
(^) Burning of fossil fuel is the largest contributor to CO 2 release, currently greater than
the 2007 total of 8.4 GtC yr−1 (“GtC” stands for gigatonnes of carbon, or 10^15 grams.
Total use of fossil fuel went up steadily after about 1750, and it increased rapidly from
the late 1940s until now (Fig. 16.5). It is popular to say that use in this interval has
increased exponentially, but that is only true of some components, not the total. The
linear increase is dramatic enough, adding 0.11 GtC per year to the annual CO 2
increment since ∼1950 (Fig. 16.6), when the post-WWII global economy suddenly
caught fire (almost literally). The annual increment is currently four-times that of the
immediate post-WWII years. Virtually all of the CO 2 generated by this burning of
fossil fuel passes through the atmosphere, and currently about half of it remains there
(Fig. 16.4). The annual average atmospheric concentration at the end of 2010 was 390
ppmv (parts/million by volume) of CO 2 , and it will pass 400 ppmv very soon.
Concentration increases currently at about 1.7 to 2.5 ppmv yr−1 (2004–2010 average).
The year-to-year variation of the increase (Fig. 16.6) is not because fuel use varies
that much. It is caused by complex interactions of weather cycles with partial
removals to the ocean. Strongest increases are during El Niño years. Some ocean
areas take in CO 2 , others release it. Particularly important as a source to the
atmosphere is the eastern tropical Pacific. Reduction of upwelling by El Niños there
should actually reduce the net transfers from the ocean. Therefore, the positive effect
of El Niños on atmospheric CO 2 increase must occur on land. It is believed to be
caused by the associated tropical droughts in the western Pacific and Asia that
generally reduce terrestrial net primary production and dry the fuel for the tropical
fires common during El Niños. Since 1 ppmv = 2.125 Gt of carbon, the increase
currently averages ∼4.4 GtC yr−1, about half of estimated burning of fossil-fuel
carbon. When it was first noticed, this difference was referred to as “missing fossil-
fuel carbon”. We now know where most of it goes, and “missing” is often used for
unaccounted remainders.
Fig. 16.5 Estimated yearly global amounts of carbon dioxide released to the
atmosphere from different sources and in total. The units are carbon in CO 2 in
millions of metric tonnes. Data are from CDIAC, Carbon Dioxide Information
Analysis Center, US Dept. of Energy
(http://cdiac.ornl.gov/trends/emis/meth_reg.html.)